Transport system making one complain (4)
I believe the answer is:
rail
I believe this clue is a double definition.
'transport system' is the first definition.
(I've seen this before)
'one complain' is the second definition.
(I've seen this before)
'making' acts as a link.
(Other definitions for rail that I've seen before include "Bar; waterside bird" , "Supported bar - inveigh" , "Water bird; barrier" , "Means of transport" , "Bar of rolled steel" .)